A person may hear a sound by an air conduction method or a bone conduction method. According to the bone conduction method, a sound may be transmitted through a skull to a cochlea and then transmitted through an auditory nerve to a brain. Since a sound vibration applied to a skin surface around an ear may be directly transmitted to a cochlea through a skull instead of through the air, a hearer may hear a sound while having a minimized sound leaking to the outside.
A wearable device such as a smart watch or a smart ring may be used while being attached to a body surface of a user. According to the bone conduction method, the user may hear a sound vibration, which is output by the wearable device to a skin surface of the user, through his skull.